Object stores don't participate in transactions

[nic-hartley]

Description

When an operation involving uploading or deleting images fails midway through, the database transaction is correctly rolled back, but S3 is left in an awkward intermediate state.

To Reproduce

1. Create an exhibit with several artifacts (POST /exhibit)
2. Edit that exhibit (POST /exhibit/{id})
3. Make the exhibit fail at some point partway through (e.g. by stopping the database, or referencing a nonexistent artifact)

Expected behavior

Everything should be reverted back to how it was before the transaction

Actual behavior

"Random" artifacts will have their S3-linked images removed

Additional context

S3 doesn't actually have transaction support, so this will definitely be a challenge. We might end up having to get rid of the bulk exhibit-editing (images in S3 that aren't referenced don't hurt anything, and can be cleaned up easily; prematurely deleted images are no longer available and cause issues). We might also be able to rig something up with S3's versioning.